The present invention relates to an optical beam printer apparatus which uses a plurality of optical beams such as laser beams, for instance, and employs an optical beam combining device for using powers of these optical beams effectively.
As is described in Japanese Patent Laid-Open No. 79216/1983 (shown by FIG. 1), for instance, conventional apparatuses employing the aforesaid optical beam combining device use an optical element 3 (called a polarizing prism in general) which is designed to transmit specific linearly-polarized light (called P polarization) and reflect the other linearly-polarized light (called S polarization) vibrating in a plane intersecting perpendicularly to the above light, on the occasion when light beams from two linearly-polarized light sources (e.g. semiconductor lasers) 1 and 2 are to be combined in the same direction substantially. As is illustrated in FIG. 2 which is an enlarged view of a part shown in FIG. 1, it is possible to use two light beams effectively by making lights of P polarization 1P and S polarization 2S fall on the polarizing prism 3 and making the two lights emitted therefrom in the same direction as indicated by numeral 12 in the figure. The apparatus shown in FIG. 1 will be described in more detail in the following.
In the double-beam scanning optical system shown in FIG. 1, one beam is used for a laser printer, while the other is used as an input for reading a copy such as a document. As for a light source, two semiconductor lasers 1 and 2 having polarization characteristics are employed. In relation to a first polarizing prism 3, the semiconductor lasers are so disposed that the planes of polarization thereof are positioned in the directions P and S respectively. Since S polarization is reflected and P polarization is transmitted as described above, the two beams can be combined once into one beam. This combined beam is scanned by a scanning system 4 such as a polygonal mirror and then focused as a small spot on a photosensitive drum 8 or a copy 7 such as a document by means of a focusing optical system 5 such as an F.theta. lens. On the occasion, the beams once combined into one are resolved into two beams by a second polarizing prism 6. In this apparatus, the beam of S polarization is reflected by both of the first and second polarizing prisms 3 and 6 to be used as a beam for reading a document. Moreover, there is a laser printer apparatus to be taken as another example in which the polarizing prism is employed to combine light beams as described above. This apparatus is designed to utilize a plurality of optical beams more effectively than conventional ones, and it shows an attempt to reduce by half the power of each laser and the number of revolutions of the polygonal mirror by simultaneously scanning the laser beams obtained in the same direction in such a way as shown previously in FIG. 2 (Collected Drafts of Lectures for The 46th Meeting of JAPAN Society of Applied Physics, autumn, 1985; 3P-H-9, p. 63). The optical system of the above-stated apparatus is as shown in FIG. 3. Concretely, two semiconductor lasers 31P and 31S are employed with the planes of polarization thereof made to intersect each other perpendicularly, two laser beams are combined and outputted in the same direction by a polarizing prism 33, and these two laser beams are made to scan synchronously. In order to maintain space spots obtained from two laser beams in the sub-scanning direction at a prescribed value, on the occasion, parts of light beams combined by said polarizing prism 33 are led out to a photodetector 39 for detecting a spot spacing so as to control the mutual spacing of the spots of these two laser beams. Said photodetector is composed of two sets of divided-in-two detecting elements or one set of divided-in-four detecting elements which are designed, as shown in FIG. 3, to be irradiated by two leakage light beams 39-1 and 39-2 led out as described above and corresponding to the elements respectively.
It is important particularly in such an apparatus as described above to adjust a balance in a quantity of light between a light led to a photosensitive drum 38 and a light led to the photodetector 39, via a polygonal mirror 34. There occur such practical disadvantages that the power of an optical beam for printing becomes insufficient when an output onto said photodetector side is too large, and that the detecting capacity of the detector turns insufficient when it is too small. The quantity of light of the optical beam for detection as preferably about 5% in general.
In FIG. 3, marks DA1 and DA2 denote differential amplifiers which obtain differential outputs (A1-A2) and (A3-A4) of the aforesaid detector respectively, numeral 30 a controller which controls galvanomirrors GM1 and GM2 on the basis of the outputs of said differential amplifiers, and BD a beam detector for printing 30 a cylinder lens.
Since the polarizing prism 33 transmits P polarization and reflects S polarization, an adjustment is required to lead parts of two linearly-polarized laser beams to the photodetector by rotating the respective light-emitting surfaces of the semiconductor lasers (by inclining the respective planes of polarization of emission thereof) generally.
However, the conventional method wherein the very light-emitting surface of a laser light source is rotated to incline the plane of polarization so as to adjust the quantity of light of laser beams has the following disadvantage. The cross section of an optical beam emitted from a semiconductor laser, a source of emission of a laser light, is elliptic generally, and the elliptic cross section of the optical beam is rotated with the rotation of the aforesaid light-emitting surface. This causes a problem that the shape of a focused laser beam spot is varied with the rotation. This causes a further problem that, in a laser printer apparatus, for instance, a spacing between scanning lines is varied from a prescribed one with a change in the shape of spots of printing beams adjacent to each other, which results in such a disadvantage as blurring of a print or the like.